The invention is related to thermally curable compositions and methods of curing.
Ring opening polymerization of cyclic ethers may be achieved by the use of several known initiator systems. Typically, cyclic ethers are polymerized using cationic initiators, except for the 3-membered cyclic ethers (hereinafter sometimes referred to as “epoxides”), whose polymerization may be initiated both by cationic as well as anionic initiators. Strong protonic acids, Lewis acids, optionally in the presence of cationogens such as alkyl halide or acyl halide, are known to initiate cationic polymerization of cyclic ethers. The initiators may also be activated by an energy source. The source of energy may be thermal, or electromagnetic, such as light, e-beam, neutron beam, and the like. The initiator, once energized, will dissociate to form excited species, which then proceed to initiate the polymerization of the cyclic ether monomer.
Polymerization of cyclic ether monomers propagates by the formation of an oxonium ion, which then is subjected to a nucleophilic attack on the carbon adjacent to the oxonium ion by an oxygen of another monomer. High molecular weight polymers with narrow molecular weight distributions may be obtained by cationic polymerization of cyclic ethers. However, cyclic ether polymerizations are subject to chain transfer reactions that may limit the molecular weight and/or broaden the molecular weight distribution of the resulting polymer. Also, termination of polymerization may be effected by the addition of specific end capping agents, or it may occur due to any of the agents already present in the reaction mixture, such as the counterion of the initiator.
The thermal initiation of polymerization of cyclic ethers has been reported before. See for example J. V. Crivello, et al. J. Polym. Sci.: Part A: Polym Chem., vol. 27, pp 3951-3968 (1989) and J. V. Crivello, et al., J. Polym. Sci.: Polym. Chem. Ed., vol. 21, pp 97-109 (1983). The initiation however required copper (II) catalysts. The mechanism for the initiation has also been speculated in those publications. Crivello et al. clearly state in the publication that “ . . . other metal salts, such as those of Ni(II), Co(II), Fe(II), Ag(I), Mn(III), Cr(III) and Pd(II), have been examined as catalysts for the reduction of diaryliodonium salts, thus far only copper compounds have been found to be effective”. Also, thermal curing requires high temperatures and difficult processing conditions. Low temperature cures are also initiated by non thermal methods, such as UV or e-beam radiation. But the other forms of radiations have issues with depth penetration. Thermal, free radical initiators may also be able to activate cationic initiators at low temperatures, but are undesirable due to outgassing of decomposition products that can create voids in cured resin, reduced shelf life at room temperature, and competing reactions that can retard the initiation and curing reactions. Thus, there is a need for compositions that are stable at ambient temperatures, but may be thermally cured at temperatures less than about 100° C., preferably less than about 90° C.